Wide range high-frequency power meter



2 Sheets-Sheet 1 W. E. GUSTAFSON WIDE RANGE HIGH-FREQUENCY POWER METERINVENToR.

M rh

@www

May 1, 1951 Filed Dec.

May l, 1951 w. E. GusTAFsoN WIDE RANGE HIGH-FREQUENCY POWER METER 2Sheets-Sheet 2 Filed Dec. 1l, 1946 Sx k@ ya Afa/1504911# Patented May l,1951v VIDE RANGE HIGH-FREQUENCY POWER METER Wilbur E. Gustafson, SanDiego, Calif. Application December 11, 1946, Serial No. 715,597

2 Claims. (Cl. 171-95) (Granted under the act of March 3., 1883, as

amended April 30.

The invention described herein maybe manufactured and used by or for theGovernment forv governmental purposes without payment to me of anyroyalty thereon.

Thisinvention relates to electric power measuring means and moreparticularly to wide range, high. frequency power meter means and amethod for measuring the high frequency power output of af radio signalgenerator, transmitter or the like substantially independently of itsfrequency.

In the past the measurement of thepow'er output of a radio transmitter'producing radio energy in the higher frequencies has been commonly byintercepting the radio energy after it has been r diated from an antennaconnected with the transmitter or by applying the output from thetransmittel' toV a dummy load and through a thermocouple in series withthe load to a millivoltmeter. Measurements so obtained have beenundesi'rably erratic and inaccurate.

object of the present invention is to pro'- v-ide an electric powermeasuring instrument that receives its input directly from a source ofpower and that, within limitations, is substantially independent oi thefrequency at which the power is supplied tothe instrument.

-f Another object is to provide a means and a method for the directmeasurement of electrical power output ina high frequency range withreproducible accuracy.

A further object is to provide a practical and simpl-y operableinstrument for the measurement of high frequency electric power and thatemploys parts that are readily obtainable and that Vcan be accuratelycalibrated.

With the above and other objects in view that will be apparent to thosewho are informed inthe field of electronics from the hereinafterdescribed method of making determinations and from the followingdescription of an illustrative instrument that is shown in theaccompanying drawings, in

electrical power is supplied from any suitablesource, such. as asignal-generator or transmitter I'; oczexample; from..which. theVpoweigas radio" output from the transmitter I into a energy, wouldnormally be radiated through a switch 2 from a transmitting antenna 3.

An instrument that. embodies the present invention is shown releasablyconnectable through the switch 2 directly to the transmitter I. The

instrument essentially comprises an attenuator t and power monitor' 5.The attenuator 4 attenuates the output from the transmitter I to alinear value that is substantially independent of frequency and thatdecreases the magnitude of the power output fromthe transmitter Isufficiently so that it may be applied safely to the power monitor 5.

The attenuator 4 preferably converts the power substantially constantvoltage, absorbs the bulk of that power, taps oiT a small fraction ofthat power and attenuates the fraction of power so tapped of'so that itmay be safely applied to the power monitor E. closed herein comprises adummy load and a predetermined length of attenuating coaxial' cable 1.The dummy load 6 receives its power input through a connecting cable orconnector 8 that is.-

releasably connected through the switch 2- with the power source ortransmitter I.

An illustrative dummy load is shown in longitudinal section as a part ofFig. 2 in the accompanying drawing. The dummy load there shown is ataper terminated, non-resonant, coaxial lossy line that absorbs thepower output from the transmitter I and that serves as a phantomantenna. The power output from the transmitter l passes through switch 2to a preferably cable connector 8. The connector 8 is joined in a usualmanner, as by a radio frequency jack S or the like, to a power absorbingsection of coaxial line. The inner contact ID of the jack 9 is connectedat one end to one conductor of the cable connector 8 and at the otherend to; the. inner or center conductor I2 of the coaxial line, as byhaving its ends on eitherside of a slot I 3 spread to t snugly within asocket I4 in the fed end of the center conductor I;2 of the coaxialline.A The other conductor of the connector 8 is connected through thejack 9r to the outer conductor I5 of the coaxial line, the outer.Aconductor It of the coaxial line being grounded in this circuit.

I-n the coaxiall line a bead I5 of polystyrene or other suitableinsulator maintains the fed end of the center conductor I2 substantiallyconcentric of the outer conductor I6. The center conductor I2 extendscoaxially of the outer conductor I-B in usual manner. The opposite orterminal' 'end-f thecen'ter conductor I'2' is similarly main- Theillustrative attenuator l that is dis-` tained coaxia'lly of the outerconductor I6 by a dummy load of suitable power absorbing dielectricmaterial I'I that is interposed therebetween.

The power absorbing dielectric material Il preferably continues inContact with the center conductor I2 for a suitable distance, or over asuitable bearing surface to provide a predeter- V mined quantity ofdielectric material Il therebetween and to maintain the center conductorI2 and outer conductor IIi in stable mechanical coaxial relation withrespect to each other. The dielectric material Il tapers graduallyforwardly from this zone of contact with the inner conductor I2 radiallyoutwardly therefrom toward the outer conductor I6, substantially as'shown in the drawing, so that the line is terminated in itscharacteristic impedance and hence has no standing waves upon it. Allenergy passing down the coaxial line so terminated is absorbed at'theterminus so that there is no reilection at this point. 'I The dielectricmaterial Il is a suitable cement with ilake graphite mixed substantiallyuniformly through it in an illustrative proportion by weight of 40 and60 per cent, respectively, that is molded into place as a paste and isthen solidied by baking. The resultant assembly' provides afpreferably50 ohm characteristic impedance line with a tapered lossy dielectric.VThe outer conductor i6 preferably is provided with a 'desired pluralityof heat interchanging ns Is that project radially from its outer surfacefor the purpose of dissipating the heat from the `coaxial line whenpower of high wattage is applied thereto.

A-One or a desired plurality of probe pick-up assemblies are locatedbetween the input end of the-coaxial line load and the beginning of thetapered dielectric material I1, in any desired manner, for tapping offpower therefrom. Between the Zero or thin forward edge of the powerabsorbing dielectric material Il and the end of the center conductor I2that contains the socket I4, the outer conductor IE is tapped in one, asshown, or in a desired plurality of positions to receive a correspondingnumber of collars 20 that arepress lit, welded or are otherwise securedat their radial inner -ends to the outer conductor I so that they areimmovable with respect thereto. In each of these positions, a hollowcylindrical tube 'ZI ts within and is adapted to make sliding engagementaxially o-f the collar 20.v The tube 2| at its radially outer end issecuredv within an externally threaded connector 22 that threads intoaninternally threaded fitting connector 3i) of Va usual coaxial linefitting that joins the probe pick-up assembly with a predeterminedlength of attenuating coaxial cable 'I.

. Within, and as a part of each of the probe pick-up assemblies, a probepick-up 25 is disposed to extend into the coaxial line between the inputend of the load and the beginning of the tapered dielectric material Il.Th-e probe pick-up 25 serves as a radio frequency connector when so puton the dummy load Ei at the end where the dielectric material I1 tapersto zeroil The probe pick-up 25 is mounted in any desired mannerfso thatit can be moved into and out ofthe field in the coaxial load, such as byhaving its upper end rmly mounted within the tting connector 22 withinsulating material 23 interposed therebetween. Y y

Y The probe pick-up 25 extends axially of the tube 2| andcollar 2l) andpreferably` terminates downwardlyv in a disc 26. Av resistance isintroduced betweenY the probe pick-up 25 andthe grounded outerconductor. lthatisvery near to 4 the capacity section of the probepick-up 25, such as the disc resistor 21 or the like. The disc resistor21 is rmly mounted within the collar 20 and is apertured centrally formaking electrical connection with the probe pick-up 25 and for thesliding of the probe pick-up 25 therethrough.

When so assembled, the probe pick-up 25 is adapted for being adjusted inthe electrical relation between its disc 25 and the coaxial line innerconductor I2 as by causing the tube 2I to be moved axially within therigidly secured collar 20. The resistor 2'I sets the internal impedanceof the pick-up section preferably to very nearly 50 ohms-over a wideband of frequencies in an illustrative experimental model that isreferred to hereinafter. The upper end of the probe pick-up 25 isadapted for being engaged in any usual manner, not shown, with the innerconductor of the coaxial cable 1. The outer conductor of the coaxialcable I is connected electrically through the tting connectors 30 and 22and the tube 2| and collar 20 with the grounded outer conductor I6 ofthe coaxial line.

The coaxial cable 'I is of a predetermined length depending upon theamount of attenuation that it is to introduce into the circuit. The endof the coaxial cable 'l that is remote from the tting connectors 30 and22'is connected through a similar connecting fitting 3| with asuitablepower monitor; such as the monitor power :meter 5' or the like.The monitor power meter 5- preferably is of a direct reading type,readings being obtained from the position of a movable hand 32 upon axed scale 33. In experimental work, a monitor power meter 5 reading upto two milliwatts full scale has been found to be adequate for low powerreadings. f

The power output from the transmitter I is substantially completelyabsorbed by the attenuation of the coaxial line looking into its inputend. The attenuation of the probe pick-up 25 is inversely proportionalto an vincrease in the frequency of the power output from thetransmitter I. The attenuation of the coaxial cable I increases directlywith increase in frequency of the power that is impressed upon it. Thelength of the coaxial cable I is such that its attenuationcounterbalances. the attenuation of the probe pick-up 25 to provide forthe instrument an overall attenuation curve that is substantially flatand an attenuation that is substantially in-I dependent ofthe frequencyof the power output from the transmitter I. The impedance. of thecoaxial cable looking into the coaxial cable from its output end ortoward the diseY resistor 21, is of smaller magnitude than, but is ofsimilar taper when compared with the impedance of the coaxial linecomprising the center conductor I2 and outer conductor I6, looking intoits input end and towardthe power absorbing dielectric material I'I. Thetapered resistor or dummy load 6 is selected to have a substantially lowvariable standing `wave ratio -so that the probe pick-up 25 will bemeasuring the power at a representative point and not at a maximum orminimum value.

The adjustment ofthe probe pick-up 25, as its disc 26 is moved towardthe inner 'conductor -I2y decreases the attenuation of the attenuator'4. The adjustment of the probe pick-up 25 .so that its disc 26 is movedaway from the inner con'- ductor I2 increases the attenuation of theattenuator 4. The adjustm-entof the probe pick-up 25 with respect -tothev inner conductor I2 altersv theattenuation of the attenuator 4. infamanner that is linear with power. vonage distribution between thecoaxialv line inner'conductor I2 and the'outer conductor i6 islogarithmic since the voltage increases in logarithmic fashion and thepower increases in linear fashion as the. probe is pressed in toward theinner conductor I2.

For these reasons, a. plurality of probe pick-up assemblies vmounted toextend radially away from the coaxial line outer conductor rI6 andlocated between the input end of the load and the beginning oi the.tapered dielectric I'I provides an instrument for measuring a relativelywide range of attenuation substantially irrespective of frequency wherethe different probe discs 25 are disposed at dierent positions withrespect to the innerv conductor i2 of the coaxial line. If the positionof. one probe disc 26 with respect to the coaxial line inner conductorI2 be taken as a reference position, another probe disc 26 that ispressed in closer toward the inner conductor I2 will have a lowerimpedance than the reference probe,` which isv the condition for lowerpower reading on the monitor meter 5. A third probe pick-up that has itsdisc 2t further away from the coaxial line inner conductor I2 than thereierence probe disc will have a higher impedance than the referenceprobe and can be used for measuring higher values of power than caneither ofthe other two probes. Where three probes are used theypreferably may be mounted along a common circumference of the coaxialline outer conductor I6 and substantially at an angle of 120. withrespect to each other. Preferably, two orthree probe pick-up assemblies,with the attenuation of each set approximately eight decibels apart areused so that different power ranges can be obtained by selecting theproper proble.

As an example, one probe pick-up assembly could be adjusted so that thetotal attenuation ofthe system of connecting cables and probe may be 40decibels; a second probe pick-up assembly could be adjusted so that thetotal attenuation of' that assembly would be 48 decibels; and a thirdprobev pick-up assembly could be adjusted so that its total attenuationwould be 56 decibels. With this arrangement a wide range high fre-Vquency power meter is provided through the exercise of the presentinvention, using monitor power meter 5 having a 12V milliwatt full scalerange. The rst probe pick-up assembly would give watts scale, the secondprobe pick-up assembly would give 120 watts full scale and the third'probe pick-upV assembly would give 800 watts full scale. Preferably theimpedance of the probe pick-up is maintained rather high so that theeiect of any standing wave which might be introduced into the coaxialline by the presence of the probe therein as a reactive component willbe minimized. Even though the load may eliminate the standingf Wavenormally in the line, the presence of the probe in the line may producea standing wave. As previously stated, the line terminated in the loadpreferably has a substantially flat voltage characteristic. Unless theprobe impedance is quite high, it may result in a mismatch and' mayinfluence the voltage on the line. The dummy load 6 is tapered toprovide a good variable standing wave ratio down to 1000 megacycles. Theprobe pick-up assembly may be set at approximately 26-36 decibels at3000 megacycles. The assembly will then have a frequency correction ofapproximately 12 decibels within the frequencyrange` of from 1000 to4000 megacycles, the attenuation decreasing with increaseI 5f" indicatedthat` substantially lall 'points ha'dfaif in frequency; A properpredetermined lengthof" cable l further attenuatesthe power and alsocorrects for the variation of attenuation with frequency that isinherent in the probe pick-up.

With the proper selection of the length of the' cable I, the total.attenuation of the probe and attenuatior system c anl be held very flatwithin` the frequency range between 1000 and 4000 megas cycles asindicated by the actual experimental' curves shown in Fig. 3 vof thedrawings.'v Wheretwo probe pick-upassemblies'on the dummy load-v were4used, two power ranges were obtained" in`= the order of 50 4and 500watts full scale.l Thev broad band matching section provided by thedevice reads power within va reproducible accuracy range of' from 10 to15 per cent over'. the

cited frequency range of from 1000 to iOOIl'niegae CYCleS.

It was determined experimentally that the disci resistor 21, acting asla load resistor of"50 ohms when used with-a coaxial cable 1 having achar-- acteristic impedance of 50 ohms vand designated' feet in length,provided a substantially constantattenuation with changing frequencyove'rtlf'ie4 frequency range from 1000 to 4000 megacycl'es.

Where the power absorbing dielectric materiall I I in the taperedresistor had a resistanceof 50 ohms, a substantially constant voltageinstead of a standing wave was obtained within the co-A axial line where20 feet of coaxiall cable 'I'of` characteristic impedance of 50 ohms wasuse'd and a disc resistor 2 of `50 ohms was usedlfor matching.

operatively the power from thev transmitter I is" fed through theconnecting cable 8 to the coaxial line part of the dummy load 6 where itis'dissipated by the dielectric material Il. The probe pick-up 25 takesa small amount of energy from the transmission line with theattenuationvof thel attenuatcr 4 varying in a manner that islinear; withpower, depending upon the proximity ofthe' probe pick-up disc 26 to thecoaxial line inner conductor I2.

axial line is determined by the nearness of the of the transmissionline.

will be measuring the point in a system with a large vdifference inmaxit mumand' minimum. v The energy picked up by the probe pick-up 25vis fed through the coaxial cable 1, which further attenuates the powerso picked. up before it" reaches the actual power measuring bridge `ormonitor power meter 5 from which readings in-A` dicating the poweroutput of the transmitter. I

over the frequency'range of Afrom 1000 to 4000*,

m-egacycles are taken.

he monitor power meter 5 used as a part of. the experimentall modelreferred to` herein meas.. ured two milliwatts of power fullv scale andhad' but one scale reading. A total of' 40 or 50. decis belsattenuation. in the circuit was requiredl therefore to lproperly measurethe power output' of the radio transmitter operating over the designatedrange.

Variable standing wavevratio measurements made upon a plurality of powermeters reading in milliwatts for use as the monitor power meter Thelevel of the energy that *isl extracted by the probe pick-up 2'5 fromthew costanding wave ratio of 2 to l oi' less over the frequencyrangebetween 1000 and 4000 megacycles. The accuracy from these determinationswas plus or minus five per cent of power or better if an averagecorrection of minus five per cent is assumed with the power that isreected from the terminus of the transmission line. The dummyloadtherefore absorbs about 95 per cent ofthe power., applied to thetransmission line. For these reasons each millivvatt power meter that isto beused as the monitor 5' should be checked for variable standing waveratio over the designated frequency band before being installed for usein the present device.

Since a .probe type pick-up has afrequency correction factor ofapproximately 12 decibels over the designated frequency range, a twovmilli- Watt monitor power meter is not adapted for being usedwith allof the attenuation in the probe pick-111125'. Currently availablestandard attenuators are not adapted for use in the disclosed devicesince they have either a fixed attenuation with varying frequency or anattenuation characteristic which does not compensate forthecorrectionfactor of the probe pick-up 25.

The attenuation correction-.factor that was introduced into the citedexperimental device by feet of coaxial cable l compensated with areasonable degree of accuracy for the, correction factor of the probepick-up'i25" with" the values of the other components in the circuit ofthe experimental device as stated. The total attenuation intheexperimental device cited herein was found to bevpractically freefrom correction over the ,designatedrange It was determined furtherexperimentally that 20v feet of attenuated cable 1 could be used tocorrect for the change in attenuation of the probe pick-up as the powerfrequency ris varied. This length of cable applies substantially ingeneral because the frequency correction for the probe pick-up 25 is thesame regardless of the attenuation level that it is set for in theabsence of standing waves set up by the load. Where eachA ofa pluralityof probe pick-ups 25 is at a different distance from the lossydielectric portion I1 of the load, the different probe plates 26 will beat different positions upon or atl a different portion of any standingwave in the coaxial line; Where the dummy load 6 can be made to have avariable standing wave ratio of 1.15 to 1 or less, there is no practicalnecessity for a frequency` correction factor for each of the variousprobes.

A calibration of the cited experimental device overthe designatedfrequency range of from 1000 to 4000 meg'acycles was made fordetermining whether or'frot-lthe total attenuation of the experimentalfdevice was constant over this frequency range.

In calibr'atingjthe attenuation of the present wide range 'highfrequency power meter model, a ohmr ser resistor was introduced after asignal genera4 fr representing the transmitter I in the drawing andahead of l0 feet of attenuuating cable that is commonly designated asRG-SS-U at the input end of the device. Another 10 feet of the RG-SS-Ucable was connected to the output of the device and fed into the inputof va mixer in a radio receiver circuit. The series resistor and the twolengths of RG-38-U cable were used as padding to minimize any resonanteffect.

With this arrangement, a signal of 1000 megacycles ywas used andthelevel of wask set on gether. With this arrangement, the signal had`the same attenuation as before except that the wide range highfrequencypower meter was not in the circuit. The step attenuator of theAradio receiver was then set to give approximately 100 reading on itsdiode meter.

vice could be obtained from a predetermined attenuation curve for theradio signal generator and receivers that had been used as reference. Ifthe readings were not exactly at 100, the decibels difference wascalculated by the voltage ratio as indicated upon the diode meter andthev difference was added or subtracted from the figure obtained fromthe attenuation curve. This procedure was repeated for each 100megacycles from 1000 to 4000 megacycles and the results were plotted toprovide the curves that are showninFig. 3 of the accompanying drawings.-

' Itwill'be apparent upon an inspection of these curves that theattenuation of the probes set 'atAOfdecibels and at 50 decibelsattenuation was fiat within plus or minus 1 decibel over the fre'-quency range tested. It is believed that plus or minus 1/2 decibel isthe probable error in the attenuation measurements since some of thefluctuations in the curves may be due to measurement errors rather thanto actual changes in attenuation. This belief was substantiated in partby running control tests based upon calol rimetric methods ofdetermining power measurement.l kSources of error in the use of thepresent device may rise from resonant effects in the generator outputcircuit or in the input circuit ofv the mixer in the radio receiver withwhich the device may be connected in the making of attenuationmeasurements. The use of padding at each end of the circuit of thepresentdevice and of a series resistance adjacent the generator at theinput end of the present device as recited in the description of theIcalibration of the experimental model is believed to substantiallyovercome these sources of error.

Frequency modulation in the output from the signal generator I may causeerrors in measurement due to slight changes in theintermediatefrequencyy characteristics of the radio receiver with whichthe present device may be connected when the step attenuator is changed.This will cause apparent differences in gain between a signal of nofrequency modulation and a signal with one or two megacycles offrequency modulation. It is preferred therefore that the attenuationcurve during the calibration of the present device be made against asignal generator that has practically no frequency modulation. It hasbeen found that attenuation measurements made with a signal generatorhaving considerablefrequency modulation during calibration of thepresent instrument are more subject to error than where this frequencymodulation is not present. Error can be further avoided by properlytracking the oscillator in the radio receiver with which the device isused since error is introduced by having too small a signal from thelocal oscillator in the mixer circuit of the receiver because too smalla signal causes the output of the mixer circuit to be dependent uponboth the input signal level andthe local oscillator signal level,

Where this reading was exactly 100, then the attenuation of the de.

In the further avoidance of error, it is advisable to match the powermeasuring loads to the oscillator in the radio receiver with which thepresent device is used since otherwise the oscillator will actually putout different amounts of power for the same input power conditions.

It is to be understood that the device that has been shown in theaccompanying drawings and described has been submitted for the purposeof illustrating and describing an illustrative embodiment of the presentinvention and that limited changes and modifications may be made in thecomponents thereof and in their arrangement without departing from thescope of the present invention.

What I claim is:

1. The method of measuring the electromotive force of high frequencypower, comprising the steps of adjustably tapping off a part of theelectromotive force, attenuating the tapped oi part of theelectro-motive force in one order, attenuating the tapped off part ofthe electromotive force in an inverse order to counterbalance theearlier attenuation thereof, and measuring the resultant part of thee1ectro motive force.

2. A power meter for indicating electrical power at high frequenciesinclusive of the range between 1,000 and 4,000 megacycles, comprising acoaxial transmission line having outer and inner conductors to whichpower may be applied, means conducting power to said coaxial line, aprobe pick-up adjustably insertabie into said coaxial line for tappingroff a fraction of the applied power and having an attenuation inverselyproportional to an increase in the frequency of the applied power,resistor means between said probe pick-up and the outer conductor ofsaid coaxial line, a coaxial cable attenuator means receiving the outputfrom said probe pick-up and having an attenuation directly proportionalto an increase in the frequency of power input to said coaxial line, andvoltage monitor means receiving the output from said coaxial cable andindicating the magnitude of the applied power substantially irrespectiveof the frequency thereof.

WILBUR E. GUSTAFSON.

REFERENCES CITED The following references are of record in the le ofthis Ipatent:

UNITED STATES PATENTS Number Name Date 1,999,250 Mollath et al Apr. 30,1935 2,273,547 Von Radinger Feb. 17, 1942 2,401,205 Usselman May 28,1946 2,404,797 Hansen July 20, 1946 2,407,590 Southworth Sept. 17, 19462,409,599 Tiley Oct. 15, 1946 2,438,915 Hansen Apr. 6, 1948 2,443,637Ovrebo June 22, 1948 2,443,921 Moe June 22, 1948 2,494,722 Rosen Jan.17, 1950

